U.S. patent application number 11/665713 was filed with the patent office on 2008-08-07 for linear low density polymers having optical and processing capabilities of low density polyethyelene.
This patent application is currently assigned to TOTAL PETROCHEMICALS RESEARCH FELUY. Invention is credited to Abbas Razavi.
Application Number | 20080188632 11/665713 |
Document ID | / |
Family ID | 38121356 |
Filed Date | 2008-08-07 |
United States Patent
Application |
20080188632 |
Kind Code |
A1 |
Razavi; Abbas |
August 7, 2008 |
Linear Low Density Polymers Having Optical and Processing
Capabilities of Low Density Polyethyelene
Abstract
The present invention discloses a catalyst system that comprises
several bridged bis- or bis-tetrahydro-indenyl components having
different substitution patterns in order to prepare polymers having
a broad molecular weight distribution.
Inventors: |
Razavi; Abbas; (Mons,
BE) |
Correspondence
Address: |
FINA TECHNOLOGY INC
PO BOX 674412
HOUSTON
TX
77267-4412
US
|
Assignee: |
TOTAL PETROCHEMICALS RESEARCH
FELUY
Seneffe (Feluy)
BE
|
Family ID: |
38121356 |
Appl. No.: |
11/665713 |
Filed: |
October 24, 2005 |
PCT Filed: |
October 24, 2005 |
PCT NO: |
PCT/EP2005/055474 |
371 Date: |
March 11, 2008 |
Current U.S.
Class: |
526/90 ; 502/170;
526/351; 526/352 |
Current CPC
Class: |
C08F 210/16 20130101;
C08F 10/00 20130101; C08F 4/65916 20130101; C08F 10/00 20130101;
C08F 4/65904 20130101; C08F 210/16 20130101; C08F 4/65904 20130101;
C08F 4/65912 20130101; C08F 4/65927 20130101; C08F 2500/12
20130101; C08F 210/06 20130101; C08F 2500/10 20130101; C08F 2500/04
20130101 |
Class at
Publication: |
526/90 ; 526/351;
526/352; 502/170 |
International
Class: |
C08F 10/00 20060101
C08F010/00; B01J 31/00 20060101 B01J031/00 |
Claims
1-10. (canceled)
11. A catalyst component comprising three or more bridged
bisindenyl metallocene components that are structurally different
in that they have different substitution patterns and represented
by formula I
R''(THI).sub.2MQ.sub.2+R''(THI)'.sub.2MQ.sub.2+R''(THI)''.sub.2MQ.sub.2+
. . . (I) wherein THI represents an unsubstituted indenyl or
tetrahydro-indenyl, THI' represents a monosubstituted indenyl or
tetrahydro-indenyl, THI'' represents a di-substituted indenyl or
tetrahydroindenyl, R'' is a structural bridge between two
cyclopentadienyl rings imparting rigidity to the component, M is a
metal group 4 of the Periodic Table (Handbook of Chemistry,
76.sup.th edition) and each Q is the same or different and may be a
hydrocarbyl or hydrocarboxy radical having 1-20 carbon atoms or a
halogen.
12. The catalyst component according to claim 11 wherein each
substituent group on the indenyls or tetrahydro-indenyls, THI' and
THI'' is independently chosen from those of formula XR.sub.v in
which X is chosen from group 14, oxygen and nitrogen and each R is
the same or different and chosen from hydrogen or hydrocarbyl of
from 1 to 20 carbon atoms and v+1 is the valence of X.
13. The catalyst component according to claim 11 wherein both THI'
are substituted at position 2 with the same substituent selected
from methyl, tert-butyl, phenyl or naphthyl.
14. The catalyst component of claim 11 wherein both THI' are
substituted at positions 2 and 4 with substituent selected from two
methyls, two tert-butyls, two phenyls or two naphtyls.
15. An active catalyst system comprising the catalyst component of
claim 11 and an activating agent or an activating support.
16. The catalyst system of claim 15 wherein the activating agent is
aluminoxane.
17. A method for homo- or co-polymerising ethylene or alpha-olefins
comprising: injecting the active catalyst system of claim 15 into a
reactor; injecting monomer and optional comonomer into the reactor;
maintaining polymerisation conditions thereby obtaining
polymer.
18. The method of claim 17 wherein the monomer is ethylene or
propylene.
19. A polymer having a molecular weight distribution of from 5 to 8
obtained by the method of claim 18.
Description
[0001] The present invention discloses metallocene catalyst systems
comprising several types of indenyl or pseudo-indenyl catalyst
components. It also discloses their use in the polymerisation of
alpha-olefins.
[0002] In many applications in which polyolefins are employed, it
is desirable that the polyolefin used has good mechanical
properties. It is known that, in general, high molecular weight
polyolefins have good mechanical properties. Additionally, since
the polyolefin must usually undergo some form of processing, such
as moulding processes and extrusion processes and the like, to form
the final product, it is also desirable that the polyolefin used
has good processing properties. However, unlike the mechanical
properties of the polyolefin, its processing properties tend to
improve as its molecular weight decreases.
[0003] Polymers having good optical properties, such as high
transparency combined with good processing were typically low
density polyethylene (LDPE) resins prepared by radical initiated
polymerisation reaction. These polymers were prepared under severe
conditions of very high pressure, typically larger than 1000 bars
and up to 3000 bars, and of high temperature, typically larger than
200.degree. C. This process was not environmentally friendly as it
released unconsumed monomers into the atmosphere. The polymer
exiting the reactor was in a molten state and included monomers
that were subsequently released in the environment. In addition,
the products did not have excellent mechanical properties. It was
also difficult to control the molecular weight and the molecular
weight distribution as the polymerisation was initiated with oxygen
and/or peroxides.
[0004] Ethylene-based copolymers produced using metallocene
catalysts were introduced to the marketplace over a decade ago,
first by Exxon Chemical Company followed closely by The Dow
Chemical Company. These copolymers had densities of at most 0.910
g/cm.sup.3. Very low density polyethylene (VLDPE) resins and
ultra-low density polyethylene (ULDPE) resins produced by
conventional methods were available on the market such as for
examples Union Carbide's Flexomer.RTM. and Mitsui's Tafmer.RTM.
product lines. Metallocene-based ethylene copolymers were however
sufficiently novel to capture novel end-use applications.
[0005] Ethylene-based copolymers having densities higher than 0.910
g/cm.sup.3 were progressively introduced on the market such as for
example Dow's octene-based linear low density polyethylene (LLDPE)
and Exxon's butene- and hexene-based LLDPE. As production of
metallocene-based LLDPE (mLLDPE) was ramped up in the mid- to late
90s, the premium commanded by these products decreased compared to
conventionally produced LLDPE. The mechanical, physical, and
optical properties of mLLDPE were far superior to those of
conventional LLDPE and low density polyethylene (LDPE). Its
processability on available equipment was however very poor in
comparison to that of conventional LDPE. Resin producers and
manufacturers of processing equipment, especially blown-film
equipment, worked simultaneously to address the problem of the
difficult processability of metallocene-based polyethylene as
compared to the very easy processing of classical LDPE.
[0006] U.S. Pat. No. 5,714,427 discloses catalyst systems
comprising a mixture of 2 metallocene components that are suitable
for the polymerisation of ethylene and alpha-olefins.
[0007] Polyethylene is an inexpensive material that can be
processed and moulded into myriads of shapes with the desired
mechanical and optical properties for numerous end uses. It has a
useful balance of physical, mechanical, and optical properties, all
of which are a function of polymer structure. Polymer structure
depends upon the catalyst system and the process technology that
are used to produce the polymer.
[0008] The properties that have an impact on processability and
mechanical properties of polyethylene are: [0009] molecular weight
[0010] molecular weight distribution [0011] molecular architecture,
specifically branching, both short-chain branching (SCB) and
long-chain branching (LCB). For SCB, both the level of SCB as well
as the distribution of SCB are important for determining the
rheological and end-use properties of the polyethylene resin.
[0012] The molecular weight of a polymer has an impact on its
hardness, durability or strength. Polymers including polyethylene
comprise short chains, long chains, and chain lengths in between,
each with a different molecular weight. An average molecular weight
can be calculated, but by itself this number is virtually
meaningless. It is preferable to characterize polymers in terms of
the distribution of the chain lengths and hence in terms of
molecular weight distribution. Quantitatively, molecular weight
distribution is described by the polydispersity index, PDI. It is
the ratio Mw/Mn of the weight average molecular weight Mw to the
number average molecular weight Mn.
[0013] The MWD of LDPE, conventional LLDPE, and metallocene-based
LLDPE differ markedly. The MWD of LDPE is typically broad of from 5
to 15, that of conventional LLDPE ranges between 4 and 6, and that
of mLLDPE is of less than 4.
[0014] The primary difference between LDPE and conventional or
metallocene-based LLDPE is in type degree and distribution of
branching, both SCB and LCB.
[0015] During the production of LDPE, SCB form via the back-biting
mechanism. Mostly ethyl and butyl branches are formed. The short
chains are distributed evenly along every chain. Typical SCB
density in LDPE is of from 10 to 30 SCB/1000 backbone carbon atoms.
The regular SCB distribution results in excellent optical
properties and a low melting point.
[0016] Type and degree of short-chain branching in linear
polyethylene made using coordination catalysts are determined by
the type and level of added comonomer. Butene-1, hexene-1, or
octene-1 are the usual comonomers, resulting in formation of ethyl,
butyl, or hexyl branches, respectively.
[0017] Catalyst type determines the distribution of SCB. A
conventional LLDPE with a density of 0.918 g/cm.sup.3 has an
average of 13-15 side branches/1000 carbons that are randomly
distributed. There is inter-chain heterogeneity, with some chains
have more SCB than others. Intra-chain SCB is a function of
molecular weight: the higher the molecular weight, the lower the
frequency of SCB. As a consequence of SCB variability, the optical
properties are poor.
[0018] One of the key features of metallocene catalysts is their
ability to incorporate comonomer uniformly both intra- and
inter-molecularly. Thus mLLDPE has a uniform comonomer distribution
that is independent of molecular weight, resulting in excellent
optical properties.
[0019] During the production of LDPE long-chain branches (LCB) form
via chain transfer. A long-chain free radical can abstract a
hydrogen atom from the backbone of a nearby chain, leaving a free
radical in the interior of the chain which reacts with nearby
ethylene molecules to form a very long branch, sometimes referred
to as a T-junction. Sufficient LCB results in formation of a
polymer network. Typically there are 15 long-chain branches/1000
carbon atoms in LDPE and 10 to 50 branch points. These branch
points function as permanent cross-links, thereby resulting in the
high melt strength of LDPE due to frequent polymer-chain
entanglements, of great benefit in extrusion processes such as
blown film and extrusion coating. Reactor type also determines the
extent of LCB in LDPE. Two types of reactor can be used: autoclave
or tubular. In general LDPE produced in an autoclave reactor has a
more complex, multi-branched structure than that produced in a
tubular reactor. More LCB results in low intrinsic viscosity.
[0020] The disadvantage of LLDPE is that there is essentially no
LCB in conventional LLDPE and no or very little LCB in mLLDPE. As a
consequence, extrusion of LLDPE produced with any type of
coordination catalyst is very difficult on equipment designed for
extruding LDPE.
[0021] The disadvantage of LDPE is that, the use of peroxides to
initiate the polymerisation of LDPE resulted in residual
contamination within the polymers. The polymers produced did not
have optimal transparency and processing properties: [0022] the
processing capabilities were reduced by long chain branching;
[0023] the crystallinity was reduced by the short chain branching
formed during polymerisation by the mechanism of backbiting.
[0024] There is thus a need to improve the processing capabilities
of mLLDPE and thus to prepare resins that would combine the good
physical, mechanical and optical properties of single-site catalyst
system and the good processability of classical LDPE resins.
[0025] To obtain the best balance of mechanical and processing
properties, polyolefins must have both a high molecular weight
(HMW) component and a low molecular weight (LMW) component: such
polyolefins have either a broad molecular weight distribution
(MWD), or a multi-modal molecular weight distribution. There are
several methods for the production of polyolefins having a broad or
multimodal molecular weight distribution. The individual
polyolefins can be melt blended, or can be formed in separate
reactors in series. Use of a dual site catalyst for the production
of a bimodal polyolefin resin in a single reactor is also
known.
[0026] Chromium-based catalysts for use in polyolefin production
also tend to broaden the molecular weight distribution and can, in
some cases, produce bimodal molecular weight distribution, but
usually the low molecular part of these resins contains a
substantial amount of the co-monomer. Whilst a broadened molecular
weight distribution provides acceptable processing properties, a
bimodal molecular weight distribution can provide excellent
properties.
[0027] Ziegler-Natta catalysts are known to be capable of producing
bimodal polyethylene using two reactors in series. Typically, in a
first reactor, a low molecular weight homopolymer is formed by
reaction between hydrogen and ethylene in the presence of the
Ziegler-Natta catalyst. It is essential that excess hydrogen be
used in this process and, as a result, it is necessary to remove
all the hydrogen from the first reactor before the products are
passed to the second reactor. In the second reactor, a copolymer of
ethylene and hexene is made in order to produce a high molecular
weight polyethylene.
[0028] Metallocene catalysts are also known in the production of
polyolefins. For example, EP-A-0619325 describes a process for
preparing polyolefins having a bimodal molecular weight
distribution. In this process, a catalyst system that includes two
metallocenes is employed. The metallocenes used are, for example, a
bis(cyclopentadienyl) zirconium dichloride and an
ethylene-bis(indenyl) zirconium dichloride. By using the two
different metallocene catalysts in the same reactor, a molecular
weight distribution is obtained, which is at least bimodal. As for
Ziegler-Natta catalysts, it is also possible to use a single
metallocene catalyst system in two serially connected loop reactors
operated under different polymerising conditions.
[0029] A problem with known bimodal polyolefins is that if the
individual polyolefin components are too different in molecular
weight and density, they may not be as miscible with each other as
desired and harsh extrusion conditions or repeated extrusions are
necessary which might lead to partial degradation of the final
product and/or additional cost. The optimum mechanical, optical and
processing properties are thus not achieved in the final polyolefin
product.
[0030] There is thus a need to prepare LDPE-like polymer resins
having controlled molecular weight distribution and controlled long
chain branching as well as good optical properties and that do not
require severe polymerisation conditions of high temperature and
high pressure.
LIST OF FIGURES
[0031] FIG. 1 represents the structure of a typical bisindenyl
metallocene catalyst component.
[0032] FIG. 2 represents the structure of a typical bisindenyl
metallocene catalyst component.
[0033] FIG. 3 represents respectively a composite molecular weight
distribution wherein the mono-substituted catalyst component is
dominant (3a), a composite molecular weight distribution wherein
the unsubstituted catalyst component is dominant (3b), and a
composite molecular weight distribution wherein the
multi-substituted catalyst component is dominant.
[0034] It is an aim of the present invention to prepare a catalyst
system that polymerises ethylene or alpha-olefins under mild
conditions of temperature and pressure.
[0035] It is also an aim of the present invention to prepare a
catalyst system for the production of polymers with controlled
molecular weight distribution.
[0036] It is another aim of the present invention to prepare a
catalyst system for the production of polymers with controlled long
and short chain branching.
[0037] It is a further aim of the present invention to prepare a
catalyst system for the production of polymers with good optical
properties.
[0038] It is yet another aim of the present invention to prepare a
catalyst system for the production of polymers that are easy to
process.
[0039] Accordingly, the present invention discloses a catalyst
component that comprises three or more bridged bisindenyl
metallocene components that are structurally slightly different in
that they have different substitution patterns. They are
represented by formula I
R''(THI).sub.2MQ.sub.2+R''(THI)'.sub.2MQ.sub.2+R''(THI)''.sub.2MQ.sub.2+
. . . (I)
wherein THI represents an unsubstituted bis- or
bis-tetrahydro-indenyl, THI' represents a substituted bis- or
bis-tetrahydro-indenyl and THI'' represents a substituted bis- or
bis-tetrahydro-indenyl having a different substitution pattern than
that of THI', R'' is a structural bridge between two
cyclopentadienyl rings imparting rigidity to the component, M is a
metal group 4 of the Periodic Table (Handbook of Chemistry,
76.sup.th edition) and each Q is the same or different and may be a
hydrocarbyl or hydrocarboxy radical having 1-20 carbon atoms or a
halogen.
[0040] In this invention, THI', THI'' . . . must be differently
substituted from one another, either by the nature of the
substituents or by the position of the substituents. Typical bis-
or bis-tetrahydro-indenyl structures are represented in FIGS. 1 and
2.
[0041] Each substituent group on the bis- or
bis-tetrahydro-indenyls THI' and THI'' may be independently chosen
from those of formula XR.sub.v in which X is chosen from group 14,
oxygen and nitrogen and each R is the same or different and chosen
from hydrogen or hydrocarbyl of from 1 to 20 carbon atoms and v+1
is the valence of X. X is preferably C. If the cyclopentadienyl
ring is substituted, its substituent groups must not be so bulky as
to affect coordination of the olefin monomer to the metal M.
Substituents on the cyclopentadienyl ring preferably have R as
hydrogen or CH.sub.3.
[0042] Preferably, THI' is mono-substituted with an alkyl or aryl
group and both THI' have the same substitution pattern. More
preferably the substituent on each THI' is at position 2 and is
selected from methyl, tert-butyl, phenyl, or naphtyl.
[0043] Preferably THI'' is di-substituted with an alkyl or aryl
group and both THI'' have the same substitution pattern. More
preferably the substituents on each THI'' are at positions 2 and 4
and are selected from methyl, tert-butyl, phenyl, or naphtyl.
[0044] In a preferred embodiment according to the present
invention, THI' is mono-substituted and THI'' is
di-substituted.
[0045] Preferably, the bridge R'' that is a methylene or ethylene
or silyl bridge either substituted or unsubstituted or a diphenyl
bridge.
[0046] The metal M is preferably the same for all components and is
selected from zirconium, hafnium or titanium, most preferably
zirconium.
[0047] Suitable hydrocarbyls for Q include aryl, alkyl, alkenyl,
alkylaryl or aryl alkyl. Each Q is preferably halogen.
[0048] The respective amounts of each metallocene component are not
particularly limited and depend upon the desired properties of the
final polymers. When good mechanical properties are needed, the
high molecular weight component is essential and the catalyst
components having a large number of substituents is favoured: a
typical composite molecular weight distribution of such resin is
represented in FIG. 3c. When good processing is preferred, the low
molecular weight component is needed and the catalyst component
without substituents is favoured: a typical composite molecular
weight distribution of such resin is represented in FIG. 3b. When a
good balance of mechanical and processing properties is preferred,
all catalyst components are equally represented.
[0049] The metallocene catalyst component used in the present
invention can be prepared by any known method. A preferred
preparation method for preparing the bis- or bis-tetrahydro-indenyl
component is described in J. Org. Chem. 288, 63-67 (1985).
[0050] An active catalyst system is prepared by combining the three
or more bis-tetrahydroindenyl catalyst components with a suitable
activating agent.
[0051] The activating agent used to activate the metallocene
catalyst component can be any activating agent having an ionising
action known for this purpose such as aluminium-containing or
boron-containing compounds. The aluminium-containing compounds
comprise alumoxane, alkyl aluminium and/or Lewis acid.
[0052] The alumoxanes are well known and preferably comprise
oligomeric linear and/or cyclic alkyl alumoxanes represented by the
formula:
##STR00001##
for oligomeric, linear alumoxanes and
##STR00002##
for oligomeric, cyclic alumoxane, wherein n is 1-40, preferably
10-20, m is 3-40, preferably 3-20 and R is a C.sub.1-C.sub.8 alkyl
group and preferably methyl.
[0053] Suitable boron-containing cocatalysts may comprise a
triphenylcarbenium boronate such as
tetrakis-pentafluorophenyl-borato-triphenylcarbenium as described
in EP-A-0427696, or those of the general formula
[L'-H]+[BAr.sub.1Ar.sub.2X.sub.3X.sub.4]-- as described in
EP-A-0277004 (page 6, line 30 to page 7, line 7).
[0054] Optionally, the catalyst components can be supported on the
same or on separate supports. Preferred supports include a porous
solid support such as talc, inorganic oxides and resinous support
materials such as polyolefin. Preferably, the support material is
an inorganic oxide in its finely divided form.
[0055] Suitable inorganic oxide materials are well known in the
art. Preferably, the support is a silica support having a surface
area of from 200-700 m.sup.2/g and a pore volume of from 0.5-3
ml/g.
[0056] Alternatively, an activating support may be used, thereby
suppressing the need for an activating agent.
[0057] The amount of activating agent and metallocene usefully
employed in the preparation of the solid support catalyst can vary
over a wide range and depend upon the nature of the activating
agent.
[0058] The active catalyst system of the present invention is used
for the polymerisation of alpha-olefins. It is particularly useful
for the preparation of polyethylene or isotactic polypropylene.
[0059] The present invention also discloses a method for
polymerising ethylene or alpha-olefins that comprises the steps of:
[0060] a) injecting into a reactor a composite active catalyst
system comprising several bridged bis-tetrahydroindenyl components
having different substitution patterns and a suitable activating
agent; [0061] b) injecting a monomer and optional comonomer into
the reactor; [0062] c) maintaining under polymerisation conditions;
[0063] d) retrieving a polymer having a broad molecular weight
distribution.
[0064] Preferably the monomer is ethylene or propylene.
[0065] The comonomer can be created in situ by adding an
oligomerisation catalyst component.
[0066] In a particularly preferred embodiment of the present
method, polymerisation takes place in a single reaction zone, under
polymerising conditions in which the catalysts producing the
polymer components are simultaneously active.
[0067] Many known procedures for forming multimodal polyolefins
have employed a different reactor for forming each component. The
methods of the present invention are particularly advantageous,
since they allow for the production of improved olefin polymers
from a single reactor. This is because the catalysts employed in
the present invention are more effective than known catalysts,
particularly when utilised simultaneously in the same reactor. This
has two distinct advantages. Firstly, since only a single reactor
is required, production costs are reduced. Secondly, since the
components are all formed simultaneously, they are much more
homogeneously blended than when produced separately.
[0068] Although polymerisation in a single reactor is particularly
preferred, the catalysts employed in the present invention are
still effective in producing the required polyolefin components of
a multimodal product even when these components are produced in
separate reactors. Accordingly, in some embodiments, separate
reactors may be employed for forming some or all of the components,
if desired
[0069] Each of the three or more bis- or bis-tetrahydro-indenyl
catalyst components produces a polymer having a narrow molecular
weight distribution, each molecular weight distribution being
slightly different than the two or more others. The resulting resin
thus has a final molecular distribution that is the superposition
of three or more narrow molecular weight distributions slightly
displaced with respect to one another. Without wishing to be bound
by theory, it is believed that the fraction of high molecular
weight component in the molecular weight distribution increases
with the number of substituents on the THI. A typical composite
molecular weight distribution is represented in FIG. 3 that
represents the superposition of molecular weight distributions for
a catalyst system comprising three bridged bis-tetrahydro-indenyl
components, the left one having no substituent, the middle one
being substituted with a methyl group at position 2, the right one
being substituted with two methyl groups, respectively in positions
2 and 4. The exact shape of the molecular weight distribution is a
function of the amount of each metallocene component: for example,
in FIG. 3a, the indenyl component having one substituent is
predominant, whereas in FIG. 3b, the unsubstituted indenyl
component is predominant and in FIG. 3c, the di-substituted indenyl
component is in major amount. It is further possible to play on the
number and nature of the substituents to modify the properties of
the final polymer.
[0070] The final molecular weight distribution is in the range of 5
to 8, preferably of from 6 to 7, whereas each individual component
has a polydispersity of from 2.5 to 4.
[0071] The polyethylene obtained with the catalyst composition
according to the present invention typically have a density ranging
from 0.910 to 0.930 g/cm.sup.3 and a melt index ranging from 0.1 to
30 dg/min. Density is measured following the method of standard
test ASTM 1505 at a temperature of 23.degree. C. and melt index MI2
is measured following the method of standard test ASTM D 1238 at a
temperature of 190.degree. C. and under a load of 2.16 kg.
[0072] The resins of the present invention can be used in the
applications of classical LDPE obtained with peroxide.
[0073] The important structural attributes of polyethylene include
molecular weight, molecular weight distribution, degree and type of
branching, comonomer distribution (compositional distribution), and
degree of crystallinity.
[0074] The physical properties of polyethylene include density,
melting temperature, crystallisation temperature, heat-deflection
temperature, glass-transition temperature, moisture and gas
permeability, and other electrical and thermal properties.
[0075] The mechanical properties of polyethylene include tensile
properties such as for example strength, modulus, tensile strength
at yield, ultimate tensile strength, flexural properties such as
strength and modulus, elongation properties such as elongation at
yield and elongation at break, tear strength, stiffness, hardness,
brittleness, impact resistance, puncture resistance, and
environmental stress crack resistance (ESCR).
[0076] The optical properties of polyethylene include clarity,
haze, gloss, and colour.
[0077] The rheological properties of polyethylenes include melt
strength, intrinsic viscosity, shear viscosity, and extensional
viscosity.
[0078] These properties vary with molecular weight, density, and
molecular weight distribution as summarised in Table I.
TABLE-US-00001 TABLE I Increases Decreases Increasing Density
stiffness, ESCR, tensile strength at yield, impact strength,
melting point, haze, hardness, gas permeability. abrasion
resistance, chemical resistance, gloss. Increasing molecular
stiffness, gloss, weight tensile strength at yield, gas
permeability. impact strength, hardness, abrasion resistance,
chemical resistance, ESCR, melt strength, haze.
[0079] As density increases so does crystallinity, so it is the
degree of crystallinity that actually determines these
properties.
[0080] The molecular weight distribution also influences the
physical properties of a polyethylene. For example, at equivalent
molecular weight, a polyethylene with a narrow MWD is tougher than
a polyethylene with a broad MWD. mLLDPE makes therefore a tougher
film than a conventional LLDPE having the same molecular weight and
density. The MWD has also an effect on the organoleptic properties
of a resin because the low molecular weight components are volatile
and extractable.
[0081] More importantly, the MWD has an effect on the
processability of the resin.
[0082] Major polyethylene processing operations include extrusion,
injection moulding, blow moulding, and rotational moulding, each
requiring different resin properties. [0083] In extrusion, molten
polymer is continuously forced through a shaped die then drawn onto
take-off equipment as it cools. Pipes, fibres, blown-film or
cast-film, sheets, coating for wire, cables, or paper are extruded
in this manner. Extrusion processes require resins with some degree
of melt strength. [0084] In injection moulding, molten polymer is
injected at very high pressure into a mould where the polymer
solidifies, replicating the shape of the mould. Resins suitable for
injection moulding must have low melt viscosity in order for the
mould to be filled quickly and completely. Typically, they have a
narrow MWD and a high melt index. The melt index is determined
using the method of standard test ASTM D 1238, at a temperature of
190.degree. C. for polyethylene and under a the load of 2.16 kg for
MI2 and 21.6 kg for HLMI [0085] In blow moulding, thin-walled
hollow parts are formed, such as for example bottles or large
articles such as drums or asymmetric articles such as automotive
fuel tanks. Blow-moulding resins require high melt strength in
order to avoid sagging or shearing away during processing.
Blow-moulding resins typically have a broad MWD and a low melt
index, usually MI2 is less than 1 dg/min and HLMI is less than 10
dg/min. [0086] In rotational moulding, finely divided polymer
powder is poured into a mould that is then heated to over
300.degree. C. and slowly rotated. As the mould rotates the polymer
melts and coats the inside walls of the mould uniformly. Rotational
moulding is a low-shear process suitable for producing large,
irregularly-shaped objects.
[0087] LDPE and LLDPE resins are used mainly to prepare various
types of film. The LDPE-like resins such as prepared in the present
invention are principally used in film applications. Other
applications may include paper extrusion-coating.
[0088] The LDPE-like resins according to the present invention have
an improved rheological behaviour when compared to conventional
LDPE. Improvements include for example the good bubble stability of
LDPE plus the draw down property of LLDPE without concomitant melt
fracture.
[0089] Conventional LDPE has a very broad MWD, wherein the lower
molecular weight fraction enhances processability whereas the
higher molecular weight fraction enhances mechanical properties. In
addition the extensive LCB present in LDPE lends very large melt
strength. Branching, both SCB and LCB, lowers the crystallinity of
solid LDPE which, combined with its homogeneous inter- and
intra-molecular branching frequency, makes it a very clear resin.
Thus LDPE is noted for its easy processing, particularly in blown
film and extrusion coating, and excellent optical properties. The
low crystallinity of LDPE means however mediocre puncture
resistance, tensile strength, and tear strength. In addition, in
processing, the draw down of LDPE is poor. It is thus difficult to
down-gauge LDPE film and thus to prepare very thin final articles.
The LDPE-like resins prepared according to the present invention do
not exhibit these drawbacks: they have excellent down-gauging
capability and good tensile and tear strength as well as excellent
resistance to puncture.
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